Tungsten is an industrially significant metal finding application in a variety of fields with particular emphasis in the tooling industry. The high hardness, heat resistance and wear resistance of tungsten and its carbide form make it an ideal candidate for use in cutting tools, mining and civil engineering tools and forming tools, such as molds and punches. Cemented tungsten carbide tools, for example, account for the majority of worldwide tungsten consumption. According a 2007 United States Geological Survey, mineral deposits of tungsten resources totaled in the neighborhood of nearly 3 million tons. At current production levels, these resources will face exhaustion within the next forty years. Moreover, a handful of nations control the majority of worldwide tungsten deposits. China, for example, controls approximately 62% of tungsten deposits and accounts for 85% of ore production volume.
Given the limited supply of tungsten and its inequitable global distribution, significant resources have been invested in the development of processes for recycling scrap tungsten carbide compositions. For example, hydrometallurgy tungsten recycling processes have been developed where tungsten carbide scrap is roasted with molten sodium nitrate (NaNO3) to generate water soluble Na2WO4. The Na2SO4 undergoes conversion to several different chemical species ending in an aqueous solution of (NH4)2SO4. Ammonium paratungstate can be easily converted to tungsten oxide (WO3) by roasting and subsequently carburized to tungsten carbide (WC). This recycling process, however, demonstrates several disadvantages including numerous processing steps, high chemical consumption and high energy consumption. Therefore, profitability is limited until large scale production is achieved.
An alternative process for recycling WC scrap employs molten zinc metal. In this process, cemented carbide scrap is mixed with zinc ingots in a tray, and the mixture is heated in a furnace to liquefy the zinc. The liquefied zinc permeates the WC scrap reacting with the metallic binder phase. The zinc is subsequently volatilized leaving behind a porous WC that is crushed into powder form. This zinc treatment process also suffers from significant disadvantages. Liquefication of the zinc, for example, requires high energy consumption. More troubling, however, is the dirty state of the resulting porous WC. Zinc treatment does not remove impurities in the WC composition such as metal carbide grain growth inhibitors and metallic binder. Such impurities limit use of the recycled WC composition in the fabrication of new tooling.